A conventionally main solar battery is manufactured by, for example, diffusing impurities having the opposite conductivity type to that of a single crystal or polycrystal silicon substrate into an acceptance surface of the silicon substrate to form p-n junction, and forming an electrode on each of the acceptance surface of the silicon substrate and a back surface located on the opposite side of the acceptance surface.
The main silicon solar battery has the acceptance surface having a front electrode (often including a metal electrode referred to as a bus bar and a finger) formed thereon and made of a metal. The front electrode blocks incident sunlight, which disadvantageously loses the output of the solar battery.
Then, a so-called back contact type solar battery is developed, which is obtained by forming an electrode only on a back surface of a silicon substrate without forming an electrode on an acceptance surface of the silicon substrate. The back contact type solar battery having the acceptance surface having no electrode formed thereon can take 100% of incident sunlight into the solar battery without having a shadow loss caused by an electrode, which can fundamentally achieve high efficiency.
When the sunlight is incident on the acceptance surface of the back contact type solar battery, careers generated near the acceptance surface of the silicon substrate reach p-n junction formed on the back surface of the back contact type solar battery. The careers are collected by a p-type finger electrode and an n-type finger electrode, and taken out to the outside.
FIG. 1 shows an electrode structure of a general solar battery. An electrode includes a finger part 102 and a bus bar part 103. The finger part 102 is a collecting electrode formed for the purpose of efficiently collecting a photoelectric current generated from the solar battery without causing a resistance loss. The bus bar part 103 collects the current through the finger part 102, and functions as a ground of a tab line. FIG. 2 shows a current flow when a tab line is applied to an acceptance surface of the general solar battery cell. As shown in FIG. 2, electrons generated within a silicon substrate 201 are collected by a finger 202 adjacent to the electrons. Furthermore, a current flows into a bus bar 203 adjacent to the finger 202, and is taken out as electric power via a tab line 204. As described above, the bus bar functions to collect the current collected by the finger, and is desirably connected to as many fingers as possible. As shown in FIG. 3A, a considerable current flows also into a diffusion layer 304 directly below a finger 301 in parallel to a current 303 of a bus bar 302 from the finger 301. The current passes through the diffusion layer even if the finger 301 is disconnected in a minimal region as shown in FIG. 3B, which provides a decrease in a resistance loss between a place in which the current is generated and the bus bar.
When a finger electrode has a decreased cross-sectional area, the finger electrode has an increased serial resistance, which causes a large output loss. Thereby, the cross-sectional area is designed to be increased. That is, the height or width of the electrode are designed to be increased. However, the former requires a plurality of processes and a protracted treatment, and is further limited. The latter can reduce a serial resistance value, but causes a decrease in the acceptance area and a deterioration in surface passivation, as a result of which the output of the solar battery is decreased in many cases. The present techniques reach a ceiling as the method for providing the high power of the solar battery.
Examples of the method for decreasing the serial resistance include increasing the cross-sectional area of the finger, and shortening a finger length. FIG. 4 shows a back surface electrode structure of a general back contact cell. The back contact type solar battery has a longer finger length than that of a general solar battery having each surface having an electrode formed thereon. There is sufficient room for an improvement in the back contact type solar battery.
A back contact type solar battery is disclosed, in which a bus bar can be formed also within a substrate region by fragmenting a conductivity type region and a conductivity type electrode formed thereon in order to shorten a finger length (Patent Literature 1).